For Windows:
When you want to take a picture, hit the print screen button. Then close the game (or minimize it) and open Microsoft Photo Editor. Open the edit menu at the top, and click paste as new image. It'll come up and you can edit it or just save it.

P.S MPE is part of Microsoft Office. If you don't have it, use your Paint program. You won't be able to edit it as much, BTW.